Scaling ADC Result to a Set Range

**mark_s** · - 25th October 2015, 23:32

Hi,

You can try this idea. It will never go under 50% using 125 as a constant. Then divide your adcin by 2 this variable will be 0 to 127

Duty = 125 + adcin/2

**RossWaddell** · - 26th October 2015, 00:45

Originally Posted by mark_s

Hi,

You can try this idea. It will never go under 50% using 125 as a constant. Then divide your adcin by 2 this variable will be 0 to 127

Duty = 125 + adcin/2

Thanks Mark. Did you choose 125 because it's half of the max duty value with the prescaler and PR2 I have above?

**mark_s** · - 26th October 2015, 01:28

Yes, because you said you were using 8 bit pwm. You can change that to fit your needs.

**RossWaddell** · - 28th October 2015, 01:07

On another note, I observed that with ADCIN the readings fluctuate quite a lot and I only want to change the motor speed if the value has 'really' changed (i.e. by me). I tried this code and while it yields a rock-solid value, it doesn't go to 0: the readings go from 1-255. Any ideas why?

Code:

' PIC 16F1825 with a 10k pot attached to AN1 (the data sheet says this is the maximum impedance allowed)

#DEFINE USE_LCD_FOR_DEBUG   ; comment out for non-debug use

' Vdd      -> pin 1         -> +5V   
' RC4/Tx   -> pin 6         -> EUSART transmit (LCD)
' RA1      -> pin 12        -> trim pot input
' Vss      -> pin 14        -> GND
   
DEFINE OSC 16               ; Set oscillator 16Mhz

DEFINE ADC_BITS      8      ; Set number of bits in result
DEFINE ADC_SAMPLEUS  50     ; Set sampling time in uS (was 5)
DEFINE ADC_CLOCK     3      ; Set clock source (3=rc)

DEFINE HSER_TXSTA   20h     ; Set transmit status and control register
DEFINE HSER_BAUD    2400    ; Set baud rate

                               
' ***************************************************************
' Device Fuses
' ***************************************************************

#CONFIG
   __config _CONFIG1, _FOSC_INTOSC & _WDTE_ON & _PWRTE_ON & _MCLRE_OFF & _CP_OFF & _CPD_OFF
   __config _CONFIG2, _PLLEN_OFF & _STVREN_ON & _BORV_LO & _LVP_OFF
#ENDCONFIG

' ***************************************************************
' Initialization
' ***************************************************************

OSCCON    = %01111000       ; 16MHz internal osc

APFCON0.2 = 0               ; Tx on RC4 for LCD display
APFCON0.7 = 0               ; Rx on RC5

BAUDCON.4 = 1               ; Transmit inverted data to the Tx pin

ANSELC    = 0               ; Digital only for all PortC pins
TRISC     = 0               ; Make all PORTC pins output

TRISA     = %00000010	    ; Make all pins output except for RA1 (trim pot input)
ANSELA    = %00000010       ; Analog on PORTA.1 (AN1) only

FVRCON    = 0               ; Fixed Voltage Reference is disabled
ADCON0    = %00000101       ; ADC (analog-to-digital) is enabled on AN1 (RA1) only
PAUSEUS 20                  ; wait for the analog switch 'glitch' to die down
ADCON1.7  = 0               ; Left-justified results in 8-bits

#IFDEF USE_LCD_FOR_DEBUG
    LCD_INST   CON 254       ' instruction
    LCD_CLR    CON 1         ' Clear screen
    LCD_L1     CON 128       ' LCD line 1
    LCD_L2     CON 192       ' LCD line 2
#ENDIF

ADCInVal       VAR BYTE      ; stores ADCIN result read from trim pot
                                                           
Main:
    gosub Do_ADC
    pause 100
    #IFDEF USE_LCD_FOR_DEBUG
        ' DO SOMETHING HERE
        HSEROUT [LCD_INST, LCD_CLR]
        pause 5
        HSEROUT ["ADC val=", DEC ADCInVal, "       ", 13, 10] ; Send text followed by carriage return and linefeed
    #ENDIF
 
GOTO Main

Do_ADC:
    PAUSEUS 50              ' Wait for A/D channel acquisition time
    ADCON0.1 = 1            ' Start conversion
    
    WHILE ADCON0.1          ' Wait for it to complete
    WEND

    ADCInVal = ADRESH
return

**amgen** · - 28th October 2015, 13:14

the voltage to the pot is probably fluxuating small amount....... add a good size cap to wiper arm (1 microfarad +-) to smooth voltage.

**RossWaddell** · - 28th October 2015, 14:31

Originally Posted by amgen

the voltage to the pot is probably fluxuating small amount....... add a good size cap to wiper arm (1 microfarad +-) to smooth voltage.

I added a 1uF, 63V electrolytic cap between the middle wiper arm input to the PIC and GND, but no change. I measured the voltage on the middle wiper arm of the pot and with a reading of '255' I see 4.99V and with '1' I see 0.00V.

Am I doing something wrong with using ADRESH in 8-bit mode?

**RossWaddell** · - 28th October 2015, 16:00

Interestingly, if I switch to 10-bit resolution on ADC I get values from 5-1023. I think there must be something wrong with my config or code:

Code:

#DEFINE USE_LCD_FOR_DEBUG   ; comment out for non-debug use

' Vdd      -> pin 1         -> +5V   
' RC5/Rx   -> pin 5         -> EUSART receive
' RC4/Tx   -> pin 6         -> EUSART transmit (LCD)
' RA1      -> pin 12        -> trim pot input (1 uF electrolytic cap?)
' Vss      -> pin 14        -> GND
   
DEFINE OSC 16               ; Set oscillator 16Mhz

DEFINE ADC_BITS      10     ; Set number of bits in result
DEFINE ADC_SAMPLEUS  50     ; Set sampling time in uS (was 5)
DEFINE ADC_CLOCK     3      ; Set clock source (3=rc)

DEFINE HSER_TXSTA   20h     ; Set transmit status and control register
DEFINE HSER_BAUD    2400    ; Set baud rate

                               
' ***************************************************************
' Device Fuses
' ***************************************************************

#CONFIG
   __config _CONFIG1, _FOSC_INTOSC & _WDTE_ON & _PWRTE_ON & _MCLRE_OFF & _CP_OFF & _CPD_OFF
   __config _CONFIG2, _PLLEN_OFF & _STVREN_ON & _BORV_LO & _LVP_OFF
#ENDCONFIG

' ***************************************************************
' Initialization
' ***************************************************************

OSCCON    = %01111000       ; 16MHz internal osc

APFCON0.2 = 0               ; Tx on RC4 for LCD display
APFCON0.7 = 0               ; Rx on RC5

BAUDCON.4 = 1               ; Transmit inverted data to the Tx pin

ANSELC    = 0               ; Digital only for all PortC pins
TRISC     = 0               ; Make all PORTC pins output

TRISA     = %00000010	    ; Make all pins output except for RA1 (trim pot input)
ANSELA    = %00000010       ; Analog on PORTA.1 (AN1) only

FVRCON    = 0               ; Fixed Voltage Reference is disabled
ADCON0    = %00000101       ; ADC (analog-to-digital) is enabled on AN1 (RA1) only
PAUSEUS 20                  ; wait for the analog switch 'glitch' to die down
ADCON1.7  = 1               ; Right-justified results in 10-bits

#IFDEF USE_LCD_FOR_DEBUG
    LCD_INST   CON 254       ' instruction
    LCD_CLR    CON 1         ' Clear screen
    LCD_L1     CON 128       ' LCD line 1
    LCD_L2     CON 192       ' LCD line 2
#ENDIF

ADCInVal       VAR WORD      ; stores ADCIN result read from trim pot
compVal        VAR WORD      ; stores last-changed ADC value

GOSUB Do_ADC
compVal = ADCInVal           ; set initial compare value
                                                           
Main:
    gosub Do_ADC
    pause 100
    #IFDEF USE_LCD_FOR_DEBUG
        If (compVal > (ADCInVal + 1)) or (compVal < (ADCInVal - 1)) THEn
            ' If this method of reading ADC is as rock-solid as it appears.
            ' then I can remove the +/- 1 before doing anything; it would be
            ' IF ADCInVal <> compVal
            
            ' DO SOMETHING HERE        
            compVal = ADCInVal
        endif
        HSEROUT [LCD_INST, LCD_CLR]
        pause 5
        HSEROUT ["ADC val=", DEC ADCInVal, "       ", 13, 10] ; Send text followed by carriage return and linefeed
    #ENDIF
 
GOTO Main

Do_ADC:
    PAUSEUS 50              ' Wait for A/D channel acquisition time
                            ' (does this need to be the same as ADC_SAMPLEUS?)
    ADCON0.1 = 1            ' Start conversion
    
    WHILE ADCON0.1          ' Wait for it to complete
    WEND

    ADCInVal.HighBYTE = ADRESH
    ADCInVal.LOWBYTE = ADRESL
return

**RossWaddell** · - 28th October 2015, 16:29

If I remove the DEFINEs usually associated with the ADCIN function:

Code:

'DEFINE ADC_BITS      10     ; Set number of bits in result
'DEFINE ADC_SAMPLEUS  50     ; Set sampling time in uS (was 5)
'DEFINE ADC_CLOCK     3      ; Set clock source (3=rc)

and use Fosc/8 as the timer:

Code:

ADCON1    = %00010000       ; Left-justified results in 8-bits; Fosc/8 as timer

Then I get 0-255. If I do the same for 10-bit resolution, though, I now get 1-1023.

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